The Intestines of the Earth

Despite most of us paying little more attention to them than a quick pirouette to avoid smooshing them as they crawl on the sidewalk after a spring rain, earthworms happen to be one of the most important soil dwelling invertebrates. In fact, they’re so important that the last scientific book that Charles Darwin ever wrote was about earthworms. So, I invite you on a journey into the soil’s depths, as we get a brief glimpse into the life of Squirmin’ Herman the red wiggler worm (http://extension.illinois.edu/worms/).

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Red wiggler worm. Look at its shiny, mucus-coated skin! Image from: https://www.groworganic.com/redworms-eisenia-foetida-800-1200-lb.html

Red wigglers are a non-burrowing species of worm. They live in the top layer of soil where their primary food source, organic matter, is abundant. A red wiggler can eat around 2 times its weight in organic matter every day! On top of this, they have huge water requirements. Water is constantly secreted through their skin as a slimy mucus. This keeps their body surface moist, so that they can crawl smoothly through the soil. Their moist skin also helps them absorb oxygen and get rid of carbon dioxide, as like other earthworms, red wiggler worms breathe through their skin!

Given these high organic matter and high moisture requirements, I conducted an experiment to test how they would prioritize these two factors. That is, when given a choice between dry soil that is rich in organic matter but lacks moisture, and wet sand that is rich in moisture but lacks organic matter, which would they choose? I was quite surprised to find that a huge majority of the red wigglers I tested, around 90%, chose the dry soil.

I wondered if there was a reason they might have avoided the wet sand. Perhaps it was because, like us, red wiggler worms can sense and respond to unpleasant stimuli, and might have avoided crawling through the sand as it may have been quite scratchy and painful compared to the fluffy soil. Or, since red wigglers aren’t physically adapted to burrow, they may have been unable to crawl into the dense, wet sand, even if they might have preferred to.

I also wondered how they might cope with the dryness of the soil. I learned that almost all earthworm species can tolerate low moisture conditions through a period of inactivity known as aestivation. Some species dig deep into the soil, and tie themselves into a knot in a mucus-lined chamber to minimize water loss. Others keep their body extended, but stop all activity until conditions are more favourable. As for Squirmin’ Herman, in this experiment it seems he and his friends chose the latter option (pictured below on the right).

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Earthworm in knot in “aestivation chamber” during drought conditions. Image from: http://www.arkive.org/earthworm/lumbricus-terrestris/image-A22318.html

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Red wiggler worms extended in dry soil in a choice experiment with dry soil and wet sand. Image taken by Brent Ludwig

 

 

 

 

 

 

 

 

 

 

With that, this brief earthworm journey comes to an abrupt close. However, there are many more earthworm adventures waiting for you outside. Next time it rains, perhaps instead of dodging them, lean in to take a closer look. These underappreciated sidewalk noodles will be pleased to find a friend.

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